1. Field of the Invention
The present invention relates to a device and method for harvesting radiant energy emanated by a distant radiant energy source, particularly, to collecting the sunlight and absorbing it by a light sensitive material, medium or device. More particularly, the present invention relates to photovoltaic devices, solar cells and light detectors having light trapping microstructures or layers to improve absorption of light within the light sensitive layer, and to a method for generating electricity from sunlight thereof.
2. Description of Background Art
Conventionally, photovoltaic solar cells or light detectors employ an active photoresponsive layer that absorbs at least a portion of the electromagnetic spectrum of the light and generates charge carriers due to the photovoltaic effect. Since most photovoltaic materials absorb much more weakly in certain wavelengths than in the others, the active layer has to have at least a minimum thickness to be able to absorb most of the light to which the photovoltaic material is responsive.
One exemplary material suitable for converting light into electricity is silicon (Si). However, Si is an indirect bandgap semiconductor and is poorly absorbing the long wavelength light. For the active layer made with crystalline silicon the minimum thickness is typically between 200 and 400 μm (micrometers). While Si is very abundant, stable and well-suited for solar cell and light detector manufacturing, the cost of this thick layer of silicon is quite high which results in the high cost of the devices.
Some other than crystalline silicon types of photovoltaic devices, such as amorphous silicon thin-film cells, for example, allow for a much smaller thickness of the active layer. However, with certain wavelengths being still absorbed very weakly, they usually require some form of light trapping that would cause the incident light to pass through the active layer multiple times thus improving the absorption. The light trapping is usually implemented in the prior art by texturing one or more surfaces comprising the solar cell in order to scatter the incident light at different angles thus resulting in a longer average light path through the active layer. In case of a monocrystalline silicon cell, light scattering and trapping is conventionally provided by microstructures such as periodic or random pyramids on the front surface and a reflective or light scattering surface at the rear of the cell. In case of an amorphous thin-film Si cell consisting of several layers, a transparent top conductor layer is often textured to scatter light and hence increase the light path through the active layer.
During light trapping, some scattered light can be trapped in the active layer of the solar photovoltaic device by means of TIR which can even allow for the multiple passage of a portion of solar rays through the active layer thus resulting in a better absorption and sunlight conversion. However, the existing approaches for light trapping in the photovoltaic devices cannot prevent for a substantial portion of incident light to escape from the device without being absorbed. For example, in case of the front surface employing random pyramidal microstructures, a large portion of the escaping light is usually lost through this front surface due to the random nature of the secondary interactions of the light rays with the pyramids. Furthermore, up to 10 percent or more light can be lost in conventional photovoltaic systems due to the reflection from front contacts or absorption by layers or surfaces which produce no photovoltaic effect.
An additional problem encountered in photovoltaic devices is that most photovoltaic materials have a relatively large refractive index which results in poor light coupling efficiency due to the high reflection losses from the light receiving surface. The bulk crystalline Si, for example, has the refractive index of 3.57 at 1,000 nm (nanometers) and 5.59 at 400 nm which results in the Fresnel reflection of 32% to 49% of the incident light at 1,000 nm and 400 nm, respectively. Typically, these problems can be addressed by adding an antireflective layer to the light receiving surface and/or surface microstructuring. However, the antireflective coating works efficiently only in a limited bandwidth and adds system cost and processing time, while the microstructures are still somewhat inefficient for light coupling or otherwise are quite expensive to be used for mass production, considering that the entire area of the photovoltaic device must be processed to cover it with these microstructures.
These drawbacks of the prior art approaches and the loss of useful light are hampering the utility of the photovoltaic devices. None of the previous efforts provides an efficient solution for coupling and trapping essentially all of the incident light and allowing it to pass through the sufficient effective depth of photosensitive material or allow the light to interact with the active layer as many times as necessary to cause the efficient light absorption in a controlled manner. It is therefore an object of this invention to provide an improved light harvesting system employing a novel photovoltaic structure with efficient light coupling and trapping thus minimizing energy loss.
The present invention solves the above problems by providing a layered structure having correlated surface relief features or microstructures that allow for enhancing the light coupling efficiency, increasing the light path through the photosensitive material and for trapping the incident light within the device by means of at least TIR. The light trapping causes multiple passage of the trapped light through the photoresponsive (active) layer thus improving the light absorption and energy conversion efficiency. Other objects and advantages of this invention will be apparent to those skilled in the art from the following disclosure.